controlled rectifiers important definitions · the load may be purely resistive, inductive or a...

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POWER ELECTRONICS UNIT-II CONTROLLED RECTIFIERS

Controlled Rectifiers

Important Definitions

Firing Angle ( Delay Angle ): Firing angle is the reference to the voltage level at which the

SCR is turned ON. Or The Firing Angle is the angle at which thyristors are triggered after

zero crossing.

Conduction Angle: The Period of Positive Half-Cycle of AC wave during which a Silicon-

Controlled Rectifier (SCR) is turned ON.

If α is the Firing angle, the conduction angle is π – α

Note : The Following points must be kept in mind while discussing controlled rectifier:

1. The necessary condition for turn ON of SCR is that, it should be forward biased and

gate signal must be applied. In other words, an SCR will only get turned ON when it is

forward biased and fired or gated.

2. SCR will only turn off when current through it reaches below holding current and

reverse voltage is applied for a time period more than the SCR turn off time.

Controlled Rectifier: “ A controlled rectifier is a circuit which is used for converting AC

Supply into Controlled DC supply & fed to the load.”

This process of converting alternating current (AC) to direct current (DC) is also called

as controlled rectification

In controlled rectifier, the diodes are replaced by Thyristors or SCRs

(Silicon Controlled Rectifiers). As the diodes offer no control over the o/p voltage, so the

Thyristors can be used to the controlled output voltage by adjusting the firing angle or delay.

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Types of Controlled Rectifier:

The phase controlled rectifier is classified into two types based on the type of input

power supply.

Single Phase Half Wave Controlled Rectifier:

Single Phase Half Wave Controlled Rectifier is a rectifier circuit which converts AC

input into “controlled” DC output only for Positive Half Cycle of the AC input supply.

The word “controlled” means that, we can change the starting point of load current by

controlling the firing angle of SCR.

A Single Phase Half Wave Controlled Rectifier circuit consists by one SCR / thyristor,

an AC voltage source and load. The load may be purely resistive, Inductive or a combination

of resistance and inductance.

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Single Phase Half Wave Controlled Rectifier with pure resistive ( R ) Load

Figure Shows the circuit diagram of Single Phase Half Wave Controlled Rectifier with

Resistive Load. In this Circuit, an SCR ( T ) is used to rectify the incoming Sine Wave from

the Input, and this rectified output will be supplied to an Resistive load.

V0 = Load output voltage

i0 = Load current

VT = Voltage across the Thyristor

During the Positive Half Cycle of the Input Supply, the SCR ( T ) is forward biased.

The load output voltage is zero till SCR triggered. During this cycle, the SCR is Triggered at a

firing angle ωt = α and SCR ( T ) will Start conducting. But as soon as the supply voltage

becomes zero at ωt = π, the load current will become zero

After ωt = π ( During Negative Half Cycle ), SCR ( T ) is reversed biased and will

Turned OFF at ωt = π and will remain in OFF condition till it is fired again at ωt = (2π+α).

The wave shapes for voltage and current in case of Resistive load are shown below:

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Therefore, the load output voltage and current for one complete cycle of input supply voltage

may be written as

v0 = Vm Sin ωt for α ≤ ωt ≤ π

i0 = Vm Sinωt / R for α ≤ ωt ≤ π

Calculation of Average Load Output Voltage:

As we know that, average value of any function f(x) cab be calculated using the formula

Let us now calculate the average value of output voltage for Single Phase Half Wave

Controlled Rectifier.

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From the expression of average output voltage, it can be seen that, by changing firing

angle α, we can change the average output voltage.

The average output voltage is maximum when firing angle is zero and it is

minimum when firing angle α = π. This is the reason, it is called phase

controlled rectifier.

Average load current for Single Phase Half Wave Controlled Rectifier can easily be

calculated by dividing the average load output voltage by load resistance R.

Single Phase Half-Wave Controlled Rectifier with Inductive-Load

Figure Shows 1 (a) the circuit diagram of Single Phase Half Wave Controlled Rectifier

with Inductive Load. In this Circuit, an SCR ( T ) is used to rectify the incoming Sine Wave

from the transformer secondary, and this rectified output will be supplied to an inductive load,

such as a motor winding or relay coil.

The wave shapes for voltage and current in case of an inductive load are given in

Fig.1.b. The load is assumed to be highly inductive.

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During the Positive Half Cycle of the Input Supply, the SCR ( T ) is forward biased.

The load output voltage is zero till SCR triggered. During this cycle, the SCR is Triggered at a

firing angle ωt = α and SCR ( T ) will Start conducting. The SCR will continue conducted in

positive half cycle.

During Negative Half Cycle, when the supply voltage reverse, the SCR ( T ) is kept

conducting continuously due to the fact that current through the inductance cannot be

reduced to Zero. During negative voltage half-cycle, current will continuous flow till the

energy stored in the inductance is dissipated in the load resistor and a part of the energy is

fed back to the source.

The effect of inductive load is increased in the conduction period of SCR. Due to this

reason, effective Load Voltage and Load Current will reduced. This problem can be

resolved by connecting a Free Wheeling Diode in anti- parallel with the inductive Load.

Freewheeling Diode

Freewheeling Diode:- A freewheeling diode is basically a diode connected across the

inductive load terminals to prevent the development of high voltage across the switch. When

the inductive circuit is switched off, this diode gives a short circuit path for the flow

of inductor decay current and hence dissipation of stored energy in the inductor. This diode is

also called Flywheel or Fly-back diode.

Purpose of using Freewheeling Diode:

1. The Freewheeling Diode improves the waveform of the load current of Rectifier

circuits, inverter circuits, and chopper circuits by making it continuous.

2. The Freewheeling protect the SCRs from damage in the circuits with Inductive Load

from the excessive reverse voltage creating by the Inductive Load.

3. The Freewheeling Diode improves the Input Power Factor of Phase controlled

Rectifiers.

4. The Freewheeling diode sustains the average output voltage of the circuit with

Inductive Load.

5. It also helps to reduce Ripple components in the output signal of the circuit with

Inductive Load.

POWER ELECTRONICS

Single Phase Half-Wave Controlled

Diode


Inductive Load and Free Wheeling Diode. In this Circuit,

incoming sine wave from the transformer secondary,

to an inductive load, such as a motor winding or relay coil. The Free Wheeling Diode is

connected across the Inductive Load in reverse biasing.

The wave shapes for voltage and

Diode is shown below:

POWER ELECTRONICS UNIT-II CONTROLLED

ontrolled Rectifier with Inductive-Load and Free Wheeling


Inductive Load and Free Wheeling Diode. In this Circuit, an SCR is used to rectify the

ave from the transformer secondary, and this rectified output will be

s a motor winding or relay coil. The Free Wheeling Diode is

connected across the Inductive Load in reverse biasing.

The wave shapes for voltage and current in case of an inductive load with Freewheeling

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RECTIFIERS

and Free Wheeling


an SCR is used to rectify the

and this rectified output will be supplied

s a motor winding or relay coil. The Free Wheeling Diode is

current in case of an inductive load with Freewheeling

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During the Positive Half Cycle of the Input Supply, the SCR is forward biased. The

load output voltage is zero till SCR triggered. During this cycle, the SCR is Triggered at a

firing angle ωt = α and SCR will Start conducting. The SCR will continue conducted in

positive half cycle and allowing the current through Inductive ( R-L ) Load. The freewheeling

diode (FD) is reverse biased During this half-cycle.

During the Negative Half-Cycle , the Freewheeling Diode becomes forward biased and

the SCR will Turned OFF, the current that was previously flowing through the SCR and the

load inductance, also starts to switch OFF, which causes the inductor to develop a large

reverse voltage (positive on the bottom of the inductor, negative on the top) to try and

maintain the previous current flow. This large reverse voltage spike would ordinarily be

applied across the SCR (positive on the anode, negative on the cathode), potentially forcing

it to continue to conduct when the gate is no longer enabled, and potentially damaging the

SCR.

For this reason, a freewheeling diode (FD) is connected in parallel with the inductive

load. With FD present, the large reverse voltage that would normally develop across the load

inductance, causes FD to become forward biased, which acts like a short-circuit to clamp the

reverse voltage spike that would otherwise occur to a safe level, corresponding to the

forward voltage drop across FD and output voltage across the load will Zero during this

Negative Half Cycle and current will flow continuously as shown in voltage & Current Wave

shapes.

Single Phase Full Wave Half Controlled Rectifier with Resistive Load:

Single Phase Full Wave Half ( Semi ) Controlled Rectifier is a rectifier that convert the

AC voltage into DC voltage during both the positive and Negative half cycles.

In Half Controlled Rectifier, One SCR and one Diode conducts for positive half cycle

and other one SCR and other Diode conducts for negative half cycle to convert the AC

voltage to DC voltage.

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The Circuit Diagram Full Wave Half controlled rectifier is as shown below:

During the Positive Half Cycle of the Input VIN Signal , The Current flowing through the

path of: Upper Terminal of the Supply ( + ) , SCR1 , Load ( RL ) , D2, and back to Lower

Terminal ( - ) of the Supply .

Similarly, During the Negative Half Cycle of Input VIN, The Current flowing through the

path of : Lower Terminal ( + ) , SCR2 , Load ( RL ), D1 and back to Upper Terminal ( - ) of

the Supply.

It is clear that one SCR from the top group (SCR1 or SCR2) and its corresponding

Diode from the bottom group (D2 or D1) must conduct together for any load current to flow.

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Thus the average output voltage, VAVE is dependent on the Firing Angle α for the two

SCRs included in the Half-Controlled Rectifier as the two diodes are uncontrolled and pass

current whenever forward biased. So for any gate firing angle α, the average output voltage

is given by:

Average Output Voltage and Current

Single Phase Full Wave Fully-controlled Bridge Rectifier with Resistive Load:

A Full Wave Full controlled rectifier is a device which converts AC supply into

Controlled DC supply & This Fully controlled DC power supply fed to the load.

This process of converting alternating current (AC) into direct current (DC) is also

called as controlled rectification.

The Circuit Diagram Full Wave Full controlled rectifier is as shown below:

In the Full Wave fully-controlled rectifier configuration, the average DC load voltage is

controlled using two thyristors / SCRs per half-cycle. Thyristors SCR1 and SCR4 are fired

together as a pair during the positive half-cycle, While thyristors SCR3 and SCR2 are also

fired together as a pair during the negative half-cycle ( i.e. 180o after SCR1 and SCR4 ).

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During the Positive Half Cycle of the Input VIN Signal , The Current flowing through the

path of: Upper Terminal of the Supply ( + ) , SCR1 , Load ( RL ) , SCR4, and back to Lower

Terminal ( - ) of the Supply .

Similarly, During the Negative Half Cycle of Input VIN, The Current flowing through the

path of : Lower Terminal ( + ) , SCR3 , Load ( RL ), SCR2 and back to Upper Terminal ( - ) of

the Supply.

As with the half-controlled rectifier, the output voltage can be fully controlled by

varying the SCRs firing / delay angle (α).

Thus the expression for the average DC voltage from a single Full Wave phase fully-

controlled rectifier in its continuous conduction mode is given as:

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Full Wave Fully-controlled Bridge Rectifier With R-L Load :

In the Full Wave Fully-Controlled Rectifier Configuration, the average DC load voltage

is controlled using two thyristors / SCRs per half-cycle. Thyristors T1 and T2 are fired together

as a pair during the positive half-cycle, While thyristors T3 and T4 are also fired together as a

pair during the negative half-cycle ( i.e. 180o after T1 and T2 ).

When the load is Inductive, the Output Voltage can be Negative for part of the cycle.

This is because an inductor stores energy in its magnetic field which is later released.

The Circuit Diagram Full Wave Full controlled rectifier with R-L Load is as shown below

Operation of this mode can be divided between four modes

Mode 1 (α to π)

In positive half cycle of applied ac signal, SCR's T1 & T2 are forward biased & can be

turned on at an angle α. Load voltage is equal to positive instantaneous AC supply voltage.

The load current is positive, ripple free, constant and equal to Io. Due to positive polarity of

load voltage & load current, load inductance will store energy.

Mode 2 (π to π + α)

At wt = π, input supply is equal to zero & after π it becomes negative. But inductance

opposes any change through it. In order to maintain a constant load current & also in same

direction. A self induced emf appears across 'L' as shown. Due to this induced voltage,

SCR's T1 & T2 are forward biased in spite the negative supply voltage. The load voltage is

negative & equal to instantaneous ac supply voltage whereas load current is positive. Thus,

load acts as source & stored energy in inductance is returned back to the ac supply.

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Mode 3 (π + α to 2π)

At wt = π + α, SCR's T3 & T4 are turned on & T1, T2 are reversed bias. Thus , process

of conduction is transferred from T1,T2 to T3,T4. Load voltage again becomes positive &

energy is stored in inductor T3, T4 conduct in negative half cycle from ( π + α ) to 2 π. With

positive load voltage & load current energy gets stored.

Mode 4 (2 π to 2 π + α)

At wt = 2π, input voltage passes through zero. Inductive load will try to oppose any

change in current if in order to maintain load current constant & in the same direction.

Induced e. m. f. is Positive & maintains conducting SCR's T3 & T4 with reverse polarity also.

Thus VL is negative & equal to instantaneous AC supply voltage. Whereas load current

continues to be positive. Thus load acts as source & stored energy in inductance is returned

back to ac supply At wt = α or 2 π + α, T3 & T4 are commutated and T1,T2 are turned ON.

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Full Wave Fully-controlled Bridge Rectifier With R-L Load & Free Wheeling Diode:

In the Full Wave fully-controlled rectifier configuration, the average DC load voltage is

controlled using two thyristors / SCRs per half-cycle. Thyristors Q1 and Q4 are fired together

as a pair during the positive half-cycle, While thyristors Q2 and Q3 are also fired together as a

pair during the negative half-cycle ( i.e. 180o after Q1 and Q4 ).

When the load is inductive, the output voltage can be negative for part of the cycle.

This is because an inductor stores energy in its magnetic field which is later released.

A free-wheeling diode can be placed in the circuit to prevent the output voltage from

going negative.

The Circuit Diagram Full Wave Full controlled rectifier with R-L Load & Free Wheeling

Diode is as shown below:

When the load is inductive, the output voltage can be negative for part of the cycle. This is

because an inductor stores energy in its magnetic field which is later released. Current

continues to flow, and the same thyristors continue to conduct, until all the stored energy is

released. Since this occurs some time after the AC source voltage passes through zero, the

output voltage becomes negative for part of cycle.

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The negative part of the output voltage waveform reduces the average output voltage E0. A

free-wheeling diode can be placed in the circuit to prevent the output voltage from going

negative. When the output voltage begins to go negative, the free-wheeling diode conducts.

This maintains the output voltage at approximately zero while the energy stored in the

inductor is released. The output voltage waveform is the same as for a purely resistive load,

and the average output voltage is therefore greater than it would be without the free-wheeling

diode. The addition of a free-wheeling diode makes the output current waveform smoother.

Full Wave Full Controlled Centre Tapped Rectifier:

The full wave Full Controlled rectifier circuit consisting of two SCRs connected with

centre tapped transformer. The Circuit Diagram Full Wave Full controlled Centre Tapped

rectifier is as shown below:

During the positive half cycle of the input, SCR1 is forward biased and SCR2 is reverse

biased. By applying the proper gate signal, SCR1 is turned ON and hence load current starts

flowing through it.

During the negative half cycle of the input, SCR2 is forward biased and SCR1 is

reverse biased. With a gate triggering, SCR2 is turned ON and hence the load current flows

through the SCR2.

Therefore, by varying the triggering current to the SCRs, the average power delivered

to the load is varied.

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FILL IN THE BLANKS:

1. A …………………. is a circuit that converts AC signal into unidirectional signal.

2. Main advantage of Bridge Converter is that it does not use any……………….

3. In a Single Phase Fully Controlled Rectifier, The….……….. of an uncontrolled

Rectifier are replaced by……………..

4. In a fully controlled Rectifier, the load voltage is controlled by controlling the

………………… of the Rectifier.

5. A Single Phase Half Wave Controlled Rectifier always operates in the

……….conduction mode.

6. A Half Wave Controlled Rectifier contains……………SCRs.

7. A Single Phase Full Wave Fully Controlled Bridge Rectifier uses………SCRs.

8. A Single Phase Full Wave Half Controlled Bridge Rectifier Contains …SCRs

9. A Free Wheeling diode is used in the Controlled Rectifier with ………Load.

10. A Single Phase Full Wave Controlled Rectifier operate in ……. Quadrants.

11. Full Form of HVDC is ………………..

12. The output voltage of a controlled Rectifier is maximum, when firing angle is

………….Degree.

13. The output voltage of a controlled Rectifier is controlled by controlling firing angle

of …………….

14. A Semi converter operate in ……… quadrants and full converter operate in

………. Quadrants.

15. The use of Free Wheeling Diode to improve wave shape of ……………..

Answers:

1) Rectifier 2) Transformer 3) Diodes, SCRs

4) Firing Angle 5) Discontinuous 6) One

7) Four 8) Two 9) Inductive

10) Two 11) High Voltage Direct Current 12) Zero

13) SCRs / Thyristors 14) One, Two 15) Load Current

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FILL IN THE BLANKS:

16. Conversion of AC into DC is called ………………..

17. Controlled Rectifiers are used to convert AC into ………………..

18. ………………Rectifier contains the mixture of diodes and SCRs.

19. ……………… Rectifier uses only SCRs in Rectification circuit.

20. When Firing Angle of SCRs increased in the Rectifier, output voltage will……

21. When Conduction Angle of SCRs increased in the Rectifier, output voltage

will………….

22. The Voltage form factor of Single Phase fully Controlled Half Wave Rectifier with

Resistive Inductive Load is ……….compared to same Rectifier with Resistive

Load.

23. The Single Phase Fully Controlled Bridge Rectifier can either operate in the

………… or ………………. Conduction Mode.

24. In the continuous conduction mode, at least …….thyristors conduct at all times.

25. Free Wheeling Diode connected in………….with load in …….. bias mode.

26. When Firing Angle of SCRs in the rectifier circuit is Zero Degree, the behaves /

output of SCRs will be like as …………

27. Firing Angle is also Known as ……..Angle.

28. Full Wave Full Controlled Centre Taped Rectifiers contains ……..SCRs.

29. …………….. Controlled Rectifiers have One SCR.

30. ……..………….. are used to convert AC into controlled D. C.

Answers:

16)Rectification 17) Controlled DC 18) Half Controlled

19) Full Controlled 20) Decreased 21) Increases

22) Poor 23) Continuous, Discontinuous 24) Two

25) Parallel, Reverse 26) Simple Rectifier 27) Delay

28) Two 29) Half Wave 30) Controlled Rectifier

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TRUE / FALSE Statement:

1. In a Single Phase Full Wave Half Controlled Bridge, both the SCRs conduct in the

Half of the AC Supply simultaneously.

2. The Half Wave Rectifier is used most of the high power applications.

3. The average ( DC ) output voltage of Full Wave Rectifier is higher than Half Wave

Rectifier.

4. When the rectification components are diodes, the circuits are termed as controlled

rectifiers.

5. When the rectification components are SCRs / Thyristors, the circuits are termed

as controlled rectifiers.

6. In Half Controlled Rectifier, Two SCRs are used.

7. Full Wave Controlled Rectifier allow the power to the Load from both cycles of the

input.

8. The output voltage of controlled rectifier is controlled by variation of firing Angle of

SCRs.

9. The uncontrolled Rectifiers contains SCRs in the circuit.

10. The output voltage of the Rectifier is decreased with the increase of conduction

angle of SCRs

11. The output current can be continuous / discontinuous depending on the R / L

(Resistance / Inductance) ratio of the Load and firing angle of SCRs.

12. The output voltage of the Rectifier is decreased with the increase of Firing angle of

SCRs

13. A full Wave Rectifier can operate in Two Quadrant.

14. A Half Wave Rectifier can operate in all four Quadrant.

15. A Single Phase Full wave Half Controlled Bridge Rectifier contain four SCRs /

Thyristors.

16. The output voltage of a controlled Rectifier is maximum, when firing angle of SCRs

is Zero Degree.

17. Full Wave Full Controlled Centre Taped Rectifiers contains Four SCRs.

18. Half Controlled Rectifier contains the mixture of diodes and SCRs.

19. Firing Angle is also Known as Conduction Angle.

20. Free Wheeling Diode connected in parallel with inductive load.

Answers:

1) FALSE 2) FALSE 3) TRUE 4) FALSE

5) TRUE 6) FALSE 7) TRUE 8) TRUE

9) FALSE 10) FALSE 11) TRUE 12) TRUE

13) TRUE 14) FALSE 15) FALSE 16) TRUE

17) FALSE 18) TRUE 19) FALSE 20) TRUE

controlled rectifiers important definitions · the load may be purely resistive, inductive or a...

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